Synthesis of 17F labeled fluoroalkanes

ABSTRACT

Fluoroalkanes such as fluoromethane labeled with  17 F are produced by contacting  17 F labeled F 2  with alkanes, preferably methane, substituted or unsubstituted alkenes, or substituted or unsubstituted alkynes in the presence of a metal oxide catalyst, preferably a silver oxide catalyst, to produce the  17 F labeled fluoroalkane. The  17 F may be produced by irradiating  20 Ne with protons, preferably having an energy of about 11 MeV and produced by a cyclotron. The  17 F labeled fluoromethane or fluoroalkanes may be produced continuously. A method for determining the location of an  17 F labeled tracer includes generating an  17 F labeled fluoroalkane, administering the  17 F labeled fluoroalkane to a test subject, and scanning the test subject with a radiosensitive detector such as a positron emission tomography scanner.

CROSS REFERENCES TO RELATED APPLICATIONS

[0001] This application is a divisional of and claims priority to U.S. Ser. No. 09/845,639, filed Apr. 30, 2001, which claims priority to U.S. Provisional Application No. 60/230,780, filed Sep. 7, 2000, now abandoned, the entire disclosures of which are herein incorporated by reference.

GOVERNMENT RIGHTS

[0002] This invention was made with United States government support awarded by the following agency: NIMH P50 MH52354. The United States has certain rights in this invention.

FIELD OF THE INVENTION

[0003] This invention pertains generally to the production of ¹⁷F and to the synthesis of ¹⁷F labeled fluoromethane and other fluoroalkanes and the use of such labeled materials in positron emission tomography.

BACKGROUND OF THE INVENTION

[0004] Positron emission tomography (PET) has found wide application as a diagnostic method. Various radioisotopes have been investigated for application in PET and other diagnostic imaging methods. One of these radioisotopes is ¹⁵O. ¹⁵O(t_(1/2)=122 seconds) tracers such as ¹⁵O labeled water are currently the most commonly used tracers in PET. One advantage of ¹⁵O labeled water as a tracer is that it can be easily and reliably synthesized. A disadvantage, however, is that it has a relatively low permeability surface product such that at high flows, the signal is reduced. Eichling et al. Circ. Res. 35, 358-364 (1974); Herscovitch et al. J. Cereb. Blood Flow Metab. 7, 527-542 (1987); Renkin, E. M. Am J. Physiol. 197, 1205-1210 (1959). Additionally, tracers such as ¹⁵O labeled water are usually administered by injection and are slow to clear test subjects.

[0005] Another radioisotope that has been used is ¹⁸F (t_(1/2)=110 minutes) in tracers such as ¹⁸F labeled fluoromethane which has been used to determine regional cerebral blood flow (rBCF). Gatley, et al. Int. J. Appl. Radiat. Isot. 32, 211-214, (1981). Because of the relatively long half life of ¹⁸F, tracers labeled with this isotope are ill suited for the fast repetitions necessary for cerebral activation protocols.

[0006] The short half life of ¹⁷F (E_(β) ⁺(max)=1.74 MeV; t_(1/2)≈64 seconds) suggests that this might be a suitable radioisotope for use in PET. The short half life presents problems, however, in that tracers labeled with this isotope must be prepared quickly in order to preserve the maximum amount of ¹⁷F in a labeled compound.

[0007]¹⁷F labeled fluoromethane has been prepared by several routes. For example, ¹⁷F labeled fluoromethane has reportedly been produced by Hunsdiecker like decomposition of ¹⁷F acetyl hypofluorite and by passage of ¹⁷F labeled F₂ through CH₃HgCl. Mulholland et al. J. Nuc. Med 28, 1082, (1987). These methods do not produce ¹⁷F labeled fluoromethane in sufficient yield for practical imaging use.

[0008] Therefore, a need exits for an improved method of generating ¹⁷F and for producing labeled fluoromethane and other fluoroalkanes from it. A need also remains for improved diagnostic methods using ¹⁷F labeled fluoromethane and other fluoroalkanes.

SUMMARY OF THE INVENTION

[0009] The present invention provides ¹⁷F labeled fluoromethane and other ¹⁷F labeled fluoroalkanes, and methods for producing ¹⁷F labeled fluoromethane and other ¹⁷F labeled fluoroalkanes. The invention also provides methods of determining the location of an ¹⁷F labeled tracer.

[0010] A method of generating ¹⁷F labeled fluoroalkanes includes contacting ¹⁷F labeled F₂ with an alkane, preferably methane, a substituted or unsubstituted alkene, or a substituted or unsubstituted alkyne in the presence of a metal oxide catalyst to produce the ¹⁷F labeled fluoroalkane. In more preferred embodiments, the ¹⁷F labeled F₂ is contacted with the alkane, the substituted or unsubstituted alkene, or the substituted or unsubstituted alkyne in the presence of the metal oxide catalyst and neon.

[0011] In preferred embodiments, the ¹⁷F is generated by proton irradiation of ²⁰Ne in a target gas stream comprising neon, preferably natural neon gas. In still other embodiments, the target gas stream includes F₂ and neon gas that includes ²⁰Ne whereas in still other preferred embodiments, the target gas stream includes helium, F₂, and neon gas that includes ²⁰Ne.

[0012] In preferred embodiments, the ²⁰Ne is irradiated with protons having an energy of greater than 8 MeV, more preferably with an energy of about 11 MeV. In another preferred embodiment the protons have an energy of about 16 MeV. The protons used to irradiate ²⁰Ne are preferably generated by a cyclotron.

[0013] In another embodiment, the ¹⁷F is generated by deuteron irradiation of ¹⁶O in a target gas stream that includes O₂.

[0014] In preferred processes, the target gas stream preferably comprises less than or about 1.0 percent, more preferably less than about 0.7 percent, and most preferably less than about 0.3 percent of F₂. In still other preferred embodiments the total pressure of the target gas ranges from about 100 to about 400 psig or more preferably ranges from about 160 to about 240 psig.

[0015] In other preferred embodiments of producing ¹⁷F labeled fluoromethane, the ¹⁷F labeled F₂ is contacted with methane, and the metal oxide catalyst is silver oxide, preferably at a temperature ranging from about 200° C. to about 600° C. More preferably, the metal oxide is at a temperature ranging from about 400° C. to about 500° C., and most preferably the metal oxide is at a temperature of about 450° C.

[0016] In other more preferred embodiments of producing ¹⁷F labeled fluoroalkanes, the ¹⁷F labeled F₂ is contacted with the substituted or unsubstituted alkene or the substituted or unsubstituted alkyne and the metal oxide catalyst is silver oxide which is more preferably at a temperature ranging from about 10° C. to about 600° C. or still more preferably is at a temperature ranging from about 20° C. to about 100° C. Most preferably, the metal oxide catalyst is at a temperature of about 25° C. in the process for producing ¹⁷F labeled fluoroalkanes.

[0017] In still other preferred embodiments, the alkene or alkyne contacted with the ¹⁷F labeled F₂ is a haloalkene or haloalkyne, more preferably a fluorinated alkene or alkyne. Still more preferably, the alkene is a difluoroalkene, yet more preferably 1,1-difluoroethylene such that the ¹⁷F labeled fluoroalkane produced is ¹⁷F labeled 1,1,1,2-tetrafluoroethane where one of the fluorine atoms is an ¹⁷F fluorine atom.

[0018] In still other preferred embodiments, the ¹⁷F labeled fluoroalkane is passed through a scrubber, preferably a soda lime scrubber.

[0019] In yet other preferred embodiments of the method of generating the ¹⁷F labeled fluoroalkane, the ¹⁷F is continuously generated by continuously irradiating the target gas stream with protons and the ¹⁷F labeled fluoroalkane is continuously produced by continuously contacting the ¹⁷F labeled F₂ with the alkane, the substituted or unsubstituted alkene, or the substituted or unsubstituted alkyne, more preferably methane.

[0020] A method of determining the location of an ¹⁷F labeled tracer is also provided. The method includes generating the ¹⁷F labeled fluoroalkane according to the invention; administering the ¹⁷F labeled fluoroalkane to a test subject; and collecting scans of the test subject using a radiosensitive detector. In preferred such methods, the ¹⁷F labeled fluoroalkane is administered to the test subject by having the test subject inhale the ¹⁷F labeled fluoroalkane. In another preferred method of determining the location of an ¹⁷F labeled tracer, the ¹⁷F labeled fluoroalkane is added to a saline solution and the saline solution is administered to the test subject.

[0021] Preferred radiosensitive detectors for use in determining the location of an ¹⁷F labeled tracer such as ¹⁷F labeled fluoromethane and other ¹⁷F labeled fluoroalkanes, are selected from a scintillation detector, a Geiger counter, a positron emission tomography scanner, a single photon emission computed tomography scanner, or a solid state detector. More preferred radiosensitive detectors for use in the method of determining the location of an ¹⁷F labeled tracer are positron emission tomography scanners or cameras.

[0022] In other more preferred embodiments of producing ¹⁷F labeled fluoroalkanes, the ¹⁷F labeled F₂ is contacted with the substituted or unsubstituted alkene or the substituted or unsubstituted alkyne and the metal oxide catalyst is silver oxide which is more preferably at a temperature ranging from about 10° C. to about 600° C. or still more preferably is at a temperature ranging from about 20° C. to about 100° C. Most preferably, the metal oxide catalyst is at a temperature of about 25° C. in the process for producing ¹⁷F labeled fluoroalkanes.

[0023] In still other preferred embodiments, the alkene or alkyne contacted with the ¹⁷F labeled F₂ is a haloalkene or haloalkyne, more preferably a fluorinated alkene or alkyne. Still more preferably, the alkene is a difluoroalkene, yet more preferably 1,1-difluoroethylene such that the ¹⁷F labeled fluoroalkane produced is ¹⁷F labeled 1,1,1,2-tetrafluoroethane where one of the fluorine atoms is an ¹⁷F fluorine atom.

[0024] The invention also provides the ¹⁷F labeled fluoromethane and fluoroalkanes produced by the processes of the invention.

[0025]¹⁷F labeled organic compounds are provided and include ¹⁷F labeled fluoromethane and an ¹⁷F labeled alkane having more than 2 carbon atoms. In more preferred ¹⁷F labeled organic compounds, the alkane comprises at least two fluorine atoms and at least one of the fluorine atoms is an ¹⁷F. In still other more preferred ¹⁷F labeled organic compounds, the ¹⁷F labeled alkane is 1,1,1,2-tetrafluoroethane where at least one of the F atoms is an ¹⁷F fluorine atom.

[0026] The invention still further provides gaseous compositions that include ¹⁷F labeled fluoromethane that has an equilibrium activity of greater than or about 20 mCi. In some preferred embodiments, the equilibrium activity level is greater than or about 40 mCi whereas in still other preferred embodiments, the equilibrium activity level is greater than or about 70 mCi.

[0027] Further objects, features and advantages of the invention will be apparent from the following detailed description when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In the drawings like numerals represent like elements.

[0029]FIG. 1 is a schematic diagram illustrating various parts of an apparatus used for the continuous production of ¹⁷F labeled fluoromethane and other ¹⁷F labeled fluoroalkanes.

[0030]FIG. 2 is a diagram of a production target in which ¹⁷F labeled F₂ is produced.

[0031]FIG. 3 is a graph showing the decay of ¹⁷F as a function of time and the decay of the small amount of ¹⁸F produced as the only radionuclidic impurity.

[0032]FIG. 4 is a graph comparing the thick target yield for ²⁰Ne(p,α)¹⁷F calculated from published total cross section data (Gruhle W., Kober B., Nuclear Physics, A286, 1, 523-530 (1977)) and the measured yield using a CTI-RDS-112 cyclotron 11 MeV proton beam.

[0033]FIG. 5 is a gas chromatograph showing the radiochemical purity of ¹⁷F labeled fluoromethane where the large peak represents ¹⁷F labeled fluoromethane and the small peak represents ¹⁷F labeled CF₄ from the target as a radiochemical impurity.

[0034]FIG. 6 is a graph showing the yield of ¹⁷F labeled F₂ as measured in mCi/μA versus the carrier F₂ flow rate in μmol/minute.

[0035]FIG. 7 is a series of brain flow images obtained from a test subject after steady state inhalation of 40 mCi of ¹⁷F labeled fluoromethane by the test subject. The test subject was a rhesus monkey and the instrument was an ECAT 944 PET scanner.

DETAILED DESCRIPTION OF THE INVENTION

[0036] Generally, the invention provides ¹⁷F labeled fluoromethane and other ¹⁷F labeled fluoroalkanes, methods for producing ¹⁷F labeled fluoromethane and other ¹⁷F labeled fluoroalkanes, and methods of determining the location of a ¹⁷F labeled tracer such as with a positron emission tomography scanner.

[0037] All ranges recited herein include all combinations and subcombinations included within that range's limits. Therefore, a temperature range of from about 200° C. to about 600° C. includes ranges of from 200° C. to 600° C., of from 300° C. to 600° C., of from 250° C. to 500° C., of from 225° C. to 425° C., etc.

[0038] Positron emitting ¹⁷F (E_(β) ⁺(max)=1.74 MeV; t_(1/2)≈64 seconds) has an ideal half life for probing such diverse processes as gray matter cerebral activity or gas-phase reactions on an industrial scale. Because of the very short half life of ¹⁷F, tracers labeled with this isotope such as ¹⁷F labeled fluoromethane provide very short half life cerebral blood flow agents that have important application in PET studies of seizure and other transient brain phenomena. The shorter half life of ¹⁷F with respect to ¹⁵O permits repeat images to be obtained twice as fast with ¹⁷F labeled tracers as compared to ¹⁵O labeled tracers such as water which is presently in wide use as a tracer as described above. The ability to scan twice as fast using ¹⁷F labeled tracers allows more data to be collected in the same amount of time and should allow more test subjects to be evaluated using instruments that are currently in high demand. Additionally, ¹⁷F labeled tracers show improved signal to noise level ratios over ¹⁵O labeled tracers providing increased sensitivity. ¹⁵O tracers such as ¹⁵O labeled water are usually administered in a manner that is more invasive than are tracers such as ¹⁷F labeled fluoromethane. For example, ¹⁵O labeled tracers are usually administered by injection and arterial measurements are then made from an arterial cannulation. On the other hand, ¹⁷F labeled tracers such as fluoromethane may be administered by having a test subject inhale the tracer, and arterial measurements can be made from exhalations. Furthermore, tracers such as ¹⁷F labeled fluoromethane are rapidly cleared by exhalation and the biological half life is quite short.

[0039] Two nuclear reactions may be used to generate ¹⁷F in practical yields for use in the present invention. First, and preferably, ¹⁷F may be produced by the irradiation of ²⁰Ne in a nuclear reaction that may be written as ²⁰Ne(p,α)¹⁷F with A(EOSB) of 14 mCi/μA leaving the target with an E_(p) equal to 11 MeV where EOSB stands for end of saturation bombardment. ¹⁷F may also be produced by the deuteron irradiation of ¹⁶O in natural or other O₂ in a process that may be written ¹⁶O(d,n)¹⁷F with A(EOSB) of 125 mCi/μA and an E_(d) value of 11 MeV. Addition of small amounts of F₂ to either target gas has been found to bring the ¹⁷F activity quantitatively out of the aluminum target. The production of ¹⁷F by proton irradiation of ²⁰Ne has several advantages over the production by deuteron irradiation of ¹⁶O. First, the ¹⁷F is produced in an inert gas for improved fast radiochemistry. Second, production of ¹⁷F by proton irradiation of ²⁰Ne is readily implementable using low energy proton cyclotrons. The ²⁰Ne is generally supplied as a component of natural neon which refers to neon gas having various isotopes and which is the type of neon commonly available and sold in gas cylinders. Similarly, natural O₂ refers to oxygen that is readily available and sold in standard gas cylinders and is found as a component in the earth's atmosphere.

[0040] Because of the relatively short half life of ¹⁷F, tracers labeled with this radioisotope must be prepared quickly to maintain optimal levels of ¹⁷F in the tracer. ¹⁷F labeled fluoromethane, [¹⁷F]CH₃F, may be conveniently and quickly generated by contacting methane, methyl bromide, methyl chloride, or methyl iodide with ¹⁷F labeled F₂ in the presence of a metal oxide catalyst, preferably a silver oxide catalyst. More preferably, ¹⁷F labeled fluoroalkanes are prepared by contacting ¹⁷F labeled F₂ with methane or other alkanes in the presence of a metal oxide catalyst where the metal oxide is at a temperature ranging from about 200° C. to about 600° C. or still more preferably at a temperature of from about 400° C. to about 500° C. or yet more preferably at a temperature of about 450° C. Alkanes that may be used to prepare ¹⁷F labeled fluoroalkanes include linear, branched chain, and cyclic alkanes. Examples of such alkanes other than methane include, but are not limited to, ethane, propane, cyclopropane, butane, cyclobutane, methylcyclopropane, 2-methylpropane, 2-methylbutane, 2,3-dimethylbutane, methylcyclobutane, pentane, cyclopentane, hexane, methylcyclopentane, cyclohexane, and mixtures thereof.

[0041] Other ¹⁷F labeled fluoroalkanes may be generated by contacting ¹⁷F labeled F₂ with a substituted or unsubstituted alkene or alkyne with a metal oxide catalyst, preferably a silver oxide catalyst. In such processes, the metal oxide catalyst is preferably at a temperature ranging from about 10° C. to about 600° C. or more preferably is at a temperature of from about 20° C. to about 100° C. Most preferably in processes using alkenes or alkynes, the metal oxide catalyst is at a temperature of about 25° C. In particular, the preparation of a ¹⁷F labeled 1,1,1,2-tetrafluoroalkane may be accomplished by contacting ¹⁷F labeled F₂ with 1,1-difluoroethylene in the presence of the metal oxide catalyst. The ¹⁷F labeled fluoromethane and other fluoroalkanes may be used as tracer compounds in positron emission tomography and in the study of industrial gases such as 1,1,1,2-tetrafluoroethane-a commercially important refrigerant. The process for generating ¹⁷F labeled F₂, ¹⁷F labeled fluoromethane, and other ¹⁷F labeled fluoroalkanes is better explained with reference to FIG. 1.

[0042]FIG. 1 is a schematic diagram of an apparatus 10 that may be used in the continuous production of ¹⁷F labeled fluoromethane and other ¹⁷F labeled fluoroalkanes. As shown in FIG. 1, the target gas, preferably ²⁰Ne in natural neon gas, is supplied to the system from a target gas cylinder 20 with a metering valve 30 through line 40 whereas F₂ is supplied to the system from a gas cylinder 50 with a metering valve 60 through line 70. The F₂ is most preferably supplied as a component of a gas mixture so that it is in a diluted rather than in a pure form. Thus, gas cylinder 50 generally supplies a mixture of gases to the system where less than about 5 percent of the gas mixture is F₂. In one preferred embodiment, the remainder of gas in cylinder 50 is an inert gas such as helium. In another preferred embodiment the remainder of gas in cylinder 50 is neon. The target gas from cylinder 20 is mixed with the gas mixture from cylinder 50 forming a target gas stream which flows through line 70 to target 80 shown in greater detail in FIG. 2.

[0043] One skilled in the art will recognize that it is not necessary that the F₂ and neon be supplied to the target 80 from separate sources. For example, in a highly preferred embodiment, a mixture of neon and F₂ is used to supply the target gas such that the target gas in cylinder 20 comprises F₂ and a species to be irradiated, preferably ²⁰Ne in natural neon gas. Thus, in a highly preferred process, cylinder 20 comprises F₂ and natural neon which includes ²⁰Ne as one of its component isotopes of Ne. No helium is required or necessary in such a process although it may certainly be present without deleterious effect on the process.

[0044] The target gas stream preferably includes ²⁰Ne and F₂ and more preferably includes ²⁰Ne, F₂, and helium in one embodiment. Most preferably, as described above, the target gas stream comprises F₂ and natural neon gas that includes ²⁰Ne among other isotopes of Ne. The target gas stream generally is less than about 3.0 percent F₂. In most processes, however, the target gas stream is less than 1.0 percent F₂, is more preferably less than about 0.7 percent F₂, and is still more preferably less than about 0.3 percent F₂ although those skilled in the art will recognize that various levels of F₂ may be used in the process of the invention. The percentage of neon and helium in a target gas stream may vary considerably. Generally, when helium is present in the target gas stream, the percentage of neon in the target gas stream is greater than 70 percent. More preferably in helium-containing gas streams, however, the percentage of neon in the target gas stream is greater than about 85 percent. Still more preferably in helium-containing target streams, the percentage of neon in the target gas stream is greater than or about 87 percent with helium and F₂ making up the remainder of such target gas streams.

[0045] In preferred processes which do not use helium, but rather consist essentially of neon and F₂, the percentage of neon in the target gas stream is greater than 97 percent. In more preferred such processes, the percentage of neon in the target gas stream is greater than about 98 percent. In still more preferred such processes, the percentage of neon in the target gas stream is greater than about 99 percent, and still more preferably is greater than about 99.5 percent.

[0046] The pressure of the target gas stream preferably ranges from about 100 to about 400 psig although those skilled in the art will recognize that the target gas stream may be at pressures outside this range. More preferably, the total pressure of the target gas stream ranges from about 120 to about 300 psig, and still more preferably ranges from about 160 to about 240 psig. The flow rate of the target gas stream into target 80 may vary considerably, but is typically maintained at a range of from about 100 to about 300 mL/minute. More preferably, the flow rate is maintained at from about 130 to about 220 mL/minute.

[0047] In the target 80, the target gas mixture is irradiated with a stream of protons 90 having an energy of greater than 8 MeV, preferably having an energy of about 11 MeV and supplied by a cyclotron such as a CTI-RDS-112 cyclotron (not shown) or having an energy of about 16 MeV and supplied by a cyclotron such as a PET Trace cyclotron available from General Electric. Irradiation of ²⁰Ne with the protons produces ¹⁷F and gives rise to the ¹⁷F labeled F₂ used to produce ¹⁷F labeled fluoromethane and other ¹⁷F labeled fluoroalkanes.

[0048] After leaving target 80, a metering valve 100 is used to regulate the flow of the ¹⁷F labeled F₂ containing gas stream which moves through line 110 to a column 120 containing a metal oxide catalyst. Metal oxide catalysts for use in the process include, but are not limited to copper oxide, nickel oxide, and silver oxide with silver oxide being a highly preferred metal oxide catalyst. Prior to entering column 120, a reactant gas supplied from cylinder 130 and regulated with metering valve 140 is added to the gas stream containing the ¹⁷F labeled F₂ through line 150. Because the gas stream contains neon, the reactant gas preferably contacts the ¹⁷F labeled F₂ in the presence of the metal oxide catalyst and neon. For producing ¹⁷F labeled fluoromethane, the reactant gas in cylinder 130 is preferably methane, methyl bromide, methyl chloride, or methyl iodide, but is most preferably methane. For producing other ¹⁷F labeled fluoroalkanes, the reactant gas is preferably a substituted or unsubstituted alkene or alkyne such as, but not limited to substituted or unsubstituted ethylene, propylene, 1-butene, 2-butene, pentenes, hexenes, cyclopentene, cyclohexene, acetylene, 1-propyne, 1-butyne, 2-butyne, pentynes, or hexynes. The alkenes or alkynes are preferably haloalkenes or haloalkynes and are more preferably fluoroalkenes or alkynes such as difluoroalkenes. An example of a particularly preferred substituted alkene is 1,1-difluoroethylene which reacts with ¹⁷F labeled F₂ to produce ¹⁷F labeled 1,1,1,2-tetrafluoroethane.

[0049] As described above, for the production of fluoromethane, the metal oxide catalyst is preferably at a temperature ranging from about 200° C. to about 600° C., more preferably ranging from about 400° C. to about 500° C., and most preferably is at a temperature of about 450° C. When the reactant gas is a substituted or unsubstituted alkene or alkyne, the metal oxide is preferably at a temperature ranging from about 10° C. to about 600° C., but is more preferably at a temperature ranging from about 20° C. to about 100° C. Most preferably, when the reactant gas is a substituted or unsubstituted alkene or alkyne, the temperature of the metal oxide catalyst is about 25° C. or room temperature. The column containing the metal oxide catalyst may be heated, if desired, using any conventional means known to those skilled in the art such as heating tape or coils.

[0050] The reactant gas from cylinder 130 may be added to the gas stream containing the ¹⁷F labeled F₂ at various flow rates which will be recognized by those skilled in the art. However preferred flow rates generally range from about 1 to about 10 mL/minute. A particularly preferred flow rate for the reactant gas is a rate of about 3 mL/minute. The reactant gas may be added to the system in a pure or diluted form. For example, methane may be added in a pure form or as a mixture with an inert gas such as helium or neon.

[0051] As described above, the ¹⁷F labeled F₂ preferably contacts the reactant gas in the presence of the metal oxide catalyst while in column 120. After passing through column 120, the product stream containing the ¹⁷F labeled fluoromethane or other fluoroalkane passes through line 160. Line 160 preferably passes through a scrubber 170 such as a soda lime scrubber or trap to remove F₂ and HF from the ¹⁷F labeled fluoromethane or other ¹⁷F labeled fluoroalkane. Tests with KI indicator strips showed that no detectable F₂ remained in the stream following the soda lime scrubber. Other separation devices such as filter 180, can be used to remove mass contaminants to undetectable levels. One such preferred type of filter is a Waters Sep-Pak Plus C18 cartridge filter.

[0052] Typical production using the procedure and apparatus set forth above with protons having an energy of about 11 MeV at a 10-15 μA beam provided 100 to 150 mCi of ¹⁷F labeled F₂, and 35 to 50 mCi of ¹⁷F fluoromethane when methane was used as the reactant gas. The process may be used to prepare gaseous compositions that include ¹⁷F labeled fluoromethane and have an equilibrium activity of greater than or about 20 mCi, of greater than or about 40 mCi, or of greater than or about 70 mCi. Such gaseous compositions generally will also contain neon or mixtures of neon and helium.

[0053] Using the apparatus described above, ¹⁷F labeled fluoromethane was produced in yields approaching the theoretical maximum of 50 percent by the addition of methane at approximately 2 percent to a target stream containing neon, helium and 0.5 percent F₂ after the target stream had been irradiated with protons and the resulting mixture containing methane had been passed through a column of powdered silver oxide at 450° C. Gas chromatography (Porpak Q column; He carrier gas; thermal conductivity detector, electrochemical detector, and positron coincidence radiation detector) indicated that the radiochemical purity of [¹⁷F]CH₃F was greater than 90 percent with less than a 100 ppm [¹⁸F]CH₃F radionuclidic contaminant. With approximately 10 μA of 11 MeV protons on target, continuous gas flow saturated the ballast reservoir at 44 mCi at a PET scanner located 20 meters away. Anesthetized primates drawing from this continuously replenished supply of ¹⁷F labeled fluoromethane exhibited a cerebral counting rate that rapidly reached a plateau of 100 kcps (7 slices, 52 mm axial field of view, 2D, CTI 933/04 PET camera). This permitted megacount flow images in a minute. The end expiratory concentration of CH₃ ¹⁷F was monitored by a flow-through beta detector, and provided the arterial concentration sufficient for quantifying regional cerebral blood flow.

[0054] Using the apparatus described above, ¹⁷F labeled 1,1,1,2-tetrafluoroethane was produced almost quantitatively by reaction of ¹⁷F labeled F₂ with 1,1-difluoroethylene in the gas-jet reaction over silver oxide at room temperature. The radiochemical purity was found to be in excess of 80 percent as determined by gas chromatography.

[0055] The neon gas used in the apparatus described above was CP grade and obtained from Air Products. The F₂ gas was obtained as a gas mixture of 5 percent F₂ in helium from Air Products. Monel brand and stainless steel Swagelok brand fittings were used. Valves used in the apparatus included Nupro brand (M- or SS- BG or BK valves, Whitey brand SS-41S1 and 41XS1 valves, and Swagelok brand SS-S1 valves. The tubing of the various lines was typically {fraction (1/16)} inch stainless steel or polytetrafluoroethylene (PTFE). The target used was that shown in FIG. 2.

[0056] As described above, FIG. 2 shows one target 80 that may be used to generate ¹⁷F labeled F₂ from ²⁰Ne in a target gas stream. The target 80 includes an aluminum housing 190 and an aluminum end flange 200 secured to housing 190 with screws 210. Aluminum end flange 200 defines an inlet 220 through which the target gas stream is introduced into the interior of housing 190. Inlet fitting 230 is used to connect target 80 to a feed gas supplied through line 70. On the other end of target 80 is the double foil cooling and target mount assembly 240 which secures target 80. The target 80 is cooled with a water cooling line 250. Viton® brand O-rings 260 are used to seal target 80. In operation, protons generated by the cyclotron (not shown) enter the interior of the housing 190 of target 80 through Havar foil 270 of about 0.0012 inch thickness. The protons irradiate the ²⁰Ne in the interior of target 80 and convert ²⁰Ne into ¹⁷F which subsequently forms ¹⁷F labeled F₂. The gaseous product leaves target 80 by passing through gas outlet 255 which has a diameter of about 0.040 inches. The gaseous product then continues to a stainless steel gas exit tube 265 held onto target 80 by a flange 270 and O-rings 280. An outlet fitting 290 connects gas exit tube 265 to line 110 which directs the ¹⁷F labeled F₂ containing stream to column 120. A metering valve may be used to control the flow of gas from target 80 into line 110.

[0057] One skilled in the art will recognize that other flow through target geometries may be used to generate the ¹⁷F in the process of the invention. Thus, the choice of a target such as that shown in FIG. 2 is not critical to the processes for preparing ¹⁷F labeled F₂, ¹⁷F labeled fluoromethane, or other fluoroalkanes.

[0058] Additionally, one skilled in the art will recognize that ¹⁷F labeled F₂ may be produced by deuteron irradiation of ¹⁶O as described above. In such a system, neon is not necessary, but the remainder of the system is similar to that described in FIG. 1. In a system using ¹⁶O and deuteron bombardment to produce ¹⁷F labeled F₂, a mixture of F₂ and O₂, preferably natural O₂ gas, is supplied to a target and then irradiated with deuterons having an energy of greater than 1 MeV. In such a system, the gas stream leaving the target would include ¹⁷F labeled F₂ and O₂ which would then proceed to column 120 containing the metal oxide catalyst.

[0059]FIG. 3 is a graph showing the decay of ¹⁷F as a function of time. The decay analysis of ¹⁷F labeled F₂ was performed after 5 minutes and 15 μA irradiation under standard flow conditions. The calculated ¹⁸F/¹⁷F number ratio in the gas stream was determined to be 3×10⁻⁵, resulting in approximately 0.2 mCi total ¹⁸F buildup in a 60 minute experiment under these conditions, compared to the equilibrium 150-175 mCi ¹⁷F.

[0060]FIG. 4 is a graph comparing the thick target yield for ²⁰Ne(p,α)¹⁷F calculated from published total cross section data (Gruhle W., Kober B., Nuclear Physics, A286, 1, 523-530 (1977)) and the measured yield using a CTI-RDS-112 cyclotron 11 MeV proton beam. The measured yield of about 20 mCi/μA is corrected for transport decay (12 seconds), neon fraction in target gas (87%), and natural ²⁰Ne fraction (90.5%)

[0061]FIG. 5 is a gas chromatograph (Porpak Q column; He carrier gas; thermal conductivity detector, electrochemical detector, and positron coincidence radiation detector) showing the radiochemical purity of ¹⁷F labeled fluoromethane produced by the present invention. The radiochemical purity of the ¹⁷F labeled fluoromethane was determined to be 91 percent with only ¹⁷F labeled CF₄ from the target as a radiochemical impurity. Notably, when used as a tracer, CF₄ stays out of the blood when inhaled. Thus, the presence of CF₄ only impacts the dosimetry to the lungs and trachea.

[0062]FIG. 6 is a graph of the yield of ¹⁷F labeled F₂ as measured in mCi/μA as a function of carrier F₂ flow as measured in μmol/minute at a fixed Ne pressure. As shown in the graph, a maximum yield of about 15 mCi/μA occurs at a F₂ flow of about 75 μmol/minute. The decrease in yield at higher concentrations of F₂ is attributed to dilution of the ²⁰Ne due to the added helium in the helium/F₂ gas mixture.

[0063] The location of ¹⁷F labeled tracers such as ¹⁷F labeled fluoromethane and other ¹⁷F labeled fluoroalkanes can be determined using a simple method. Typically, the ¹⁷F labeled tracer is first generated using the processes of the present invention. The tracer is then administered to a test subject such as a human, animal, plant, process plant, or a chemical reaction. After the tracer has been administered to the test subject, the test subject is scanned using a radiosensitive detector. Examples of suitable radiosensitive detectors include, but are not limited to, scintillation detectors, Geiger counters, positron emission tomography scanners, single photon emission computed tomography scanners, and solid state detectors such as, but not limited to, PIN diodes, silicon-based detectors, and germanium-based detectors. A more preferred radiosensitive detector for use in the method of determining the location of an ¹⁷F labeled tracer is a positron emission tomography scanner or camera which may be used to perform positron emission tomography as described below as an example of a type of method of determining the location of an ¹⁷F labeled tracer.

[0064] Positron emission tomography can be readily accomplished using the ¹⁷F labeled fluoromethane and other ¹⁷F labeled fluoroalkanes produced by the process of the invention as tracers. Typically, the ¹⁷F labeled compound is administered to a test subject and then scans are collected on the test subject using a positron emission tomography scanner. ¹⁷F labeled fluoromethane or other ¹⁷F labeled fluoroalkanes are most preferably administered to human and animal test subjects by having the test subject inhale a gas mixture containing the ¹⁷F labeled material. Alternatively, the ¹⁷F labeled compound may be added to a saline solution and then administered to an animal or human test subject such as by intravenous injection. A condenser such as a Waters Sep-Pak Plus C18 cartridge filter may be used in conjunction with ethanol/dry ice to trap fluoroalkanes such as fluoromethane. The Waters Sep-Pak Plus C18 cartridge filter may then be rinsed with saline to produce the saline solution for administration.

[0065]FIG. 7 shows brain flow images obtained with an ECAT 933 PET scanner using ¹⁷F labeled fluoromethane as a tracer. The test subject was a rhesus monkey. The ¹⁷F labeled fluoromethane was administered by having the rhesus monkey inhale the tracer at a steady state level of 40 mCi.

[0066] It is understood that the invention is not limited to the embodiments set forth herein for illustration, but embraces all such forms thereof as come within the scope of the following claims. 

What is claimed is:
 1. A gaseous composition, comprising ¹⁷F labeled fluoromethane and having an equilibrium activity of greater than or about 20 mCi.
 2. The gaseous composition according to claim 1, wherein the equilibrium activity is greater than or about 40 mCi.
 3. The gaseous composition according to claim 1, wherein the equilibrium activity is greater than or about 70 mCi.
 4. The gaseous composition according to claim 1, further comprising neon. 